Studies of the biology of thyrotropin releasing hormone have continued. The metabolite of thyrotropin releasing hormone called histidyl-proline diketopiperazine has been the focus of our attention. We have accumulated further evidence that this compound is biologically active. This year we showed that histidyl-proline diketopiperazine is a non-competitive inhibitor of dopamine uptake into synaptosomes. An exploration of the mechanism of the inhibition of catecholamine transport by the peptide revealed that histidyl-proline diketopiperazine inhibits the sodium-potassium dependent ATPase that generates the ion-gradient necessary for catecholamine uptake.